TALLAHASSEE, Fla., Jan 7 (Reuters) - A Florida man who has spent nearly 40 years on death row for killing a Miami couple and later stabbing a prison guard to death was set to be executed by lethal injection on Tuesday, a state official said.

Askari Abdullah Muhammad, 62, who was known as Thomas Knight when he killed his former employer and his wife in 1974, won a stay of execution last month after he legally challenged the use of a sedative, midazolam hydrochloride, as the first in a series of three drugs used for lethal injections in Florida.

The state switched to midazolam last year when makers of another sedative, pentobarbital, refused to supply it to states using the drug in executions.

Two previous executions were carried out with midazolam last year. Attorneys for Muhammad alleged the new drug was an ineffective sedative and caused inmates to suffer pain when the subsequent two drugs - a paralytic agent and heart-stopping drug - were administered to complete the injection process.

Circuit Judge Phyllis Rosier ruled, however, that there was insufficient evidence of pain in the two previous executions. The Florida Supreme Court upheld her decision on Dec. 19 and the state prison system rescheduled the execution.

Muhammad is scheduled to be executed at the Florida State Prison in Starke at 6 p.m. local time (2300 GMT) on Tuesday.

He made a final appeal in late December, arguing that the state withheld evidence of his mental condition that might have disputed premeditation in the slaying of the prison guard.

The state's highest court rejected that appeal without comment on Monday.

Muhammad was sentenced to death for the 1974 murders of Sydney and Lillian Gans in Miami. Muhammad had previously worked for Gans at a paper bag company, and abducted him from a parking lot with a rifle, forcing Gans to drive home and get his wife before making them withdraw $50,000 from a bank.

He shot the couple in the back of their heads and fled, but was captured a short time later.

Muhammad, who was then known as Knight, escaped while awaiting trial and was implicated in the fatal shooting of a liquor store clerk in October 1974 in Cordele, Georgia. He was not charged in that case, but was returned to Florida for trial in the Gans murders.

In 1980, while on "Death Row," Muhammad stabbed Correctional Officer Richard Burke with a sharpened spoon while being escorted to a shower room. He was sentenced to die for Burke's killing in 1983. (Editing by Kevin Gray, G Crosse)